There's a news squib from the Institutes of Physics this morning touting new results on a theory of modified gravity that the authors say can explain the structure of the universe without needing to invoke dark matter. This is a significant problem in cosmology, as the article explains:
[O]ur theory of gravitation - Einstein's theory of general relativity - cannot account for the extent of clumping without invoking the right amount of a mysterious substance called "dark matter". Originally introduced in the 1930s to explain anomalous galaxy dynamics, dark matter (which cosmologists think could make up to 95% of matter in the universe) is gravitationally attractive yet does not couple to light. But crucially it can describe how the initial plasma fluctuations were sustained for long enough to allow large structures to form - on its own, general relativity attests that they simply petered out. Even though dark matter has never been observed, the majority of physicists now believe that general relativity combined with dark matter is the only satisfactory explanation for the universe's large-scale structure.
However, in recent years there has been growing support for alternative theories of gravitation to general relativity that do away with the need for dark matter altogether. One of these, devised by Jacob Bekenstein in 2004 at the Hebrew University of Jerusalem, uses vector and scalar fields in addition to the tensor used in relativity, hence the name "TeVeS" (Tensor Vector Scalar). TeVeS has already been shown to explain galaxy dynamics without the need for dark matter. But now, building upon the numerical studies by Pedro Ferreira and colleagues performed earlier this year, Scott Dodelson and Michele Liguori from Fermilab in the US have confirmed that TeVeS can also provide such sustained plasma fluctuations. (See figure: "Powering galaxy formation".)
On the one hand, this sounds really interesting. However, I thought the Bullet Cluster results pretty much nailed the case for dark matter some months back. And yet, it's apparently a result appearing in Physical Review Letters, a journal not known for publishing kookery (here's the PRL link, for those with institutional access).
I am confused. So I'll post it here, and hope that somebody who understands the current state of gravity theory will come along and explain this to me.
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What you need is Sean Carroll, or somebody of his ilk.
I don't really understand the theory well enough to know that this can be completely ruled out. However, at this point the MOND people seem to me to be holding on to an idea that is no longer needed, past the point where it's rational to hold on to it.
Arp was a guy who for many, many years didn't accept the cosmological redshift as the source of redshifts seen from distant galaxies. He would point to quasars next to lower redshift galaxies, assert there was a bridge or a connection, and say that quasars were smaller, closer objects ejected from those galaxies rather than enormously distant objects. He held on to this long past when he should have.
I don't think there is a non-dark-matter way of accounting for the bullet cluster results. It may still be that gravity is modified on large scales -- but given that dark matter is there, and is most of the mass, economy of explanation suggests that it makes most sense to ascribe structure formation the very function and workable models of dark matter,
-Rob
I assume that the article does not address the Bullet Cluster results...? That seems like a pretty big oversight, given that the results have been out since August -- and they are designed to specifically discount this kind of thing. I'm sure there's a long lead time in scientific publishing, but wouldn't you at least add a postscript to address it?
The article mentions in passing an earlier (2004) version of the Bullet Cluster results (Ref. 7, Clowe et al.), but it only says that there has been argument about the consistency of MOND with observations, citing studies like that one.
There is still some wriggle room for dark matter + MOND scenarios, as Rob mentions, but it's hard to justify pure modified-gravity theories of everything nowadays.
While this is an interesting area of research that should be pursued, it is still way behind dark matter. This alternate theory of gravity still cannot reproduce the CMB spectrum, although it does well on galaxy and cluster scales. It also necessitates the existence of a new, unobserved scalar field, so the complaint leveled at dark matter - that is is problematic because it requires a new particle exist in nature - can also be leveled at this theory. I hate to add that we have historically observed new particles rather often, but how many scalar fields exist in nature again?
For those interested, the full paper can be found in preprint form here:
http://xxx.lanl.gov/abs/astro-ph/0608602
I'm sorry, but this is not nearly tinfoil enough for someone with tenure....
cisko -- for what it's worth, the submission date on the astro-ph version of the article which Mike kindly linked to is 29 August, so the article was undoubtedly written and submitted prior to the recent Bullet Cluster results.
Rob -- I don't think it's fair to compare MOND proponents to Halton Arp -- at least, not yet. There were some serious problems with matching Cold Dark Matter predictions of galaxy properties to real galaxies up until a few years ago (e.g., rotation curves in the central regions of dwarf galaxies, overpredicting the number of dwarf galaxies, difficulties reproducing the Tully-Fisher relation, etc.) -- and there are still problems with some of these. It wasn't necessarily "irrational" to look at alternatives and try to develop them.
Peter -- I certainly agree it wasn't irrational at all to try and develop alternatives.
However, at some point, hanging on to an idea as it becomes less and less likely, and less and less needed, becomes irrational. Early on, there may have been some reason to doubt cosmological redshift, but Arp held on to it for too long. Once upon a time, the Steady State Universe made sense given what we knew, but Fred Hoyle hung on to it too long.
Maybe we're not quite there yet with MOND, but we're getting close. At some point, those folks are going to need to move on, or else they will be similarly irrationally holding on to an alternative theory the mainstream when there is no longer any real convincing reason to do so.
Even a year or two ago, there wasn't really good evidence separating MOND from Dark Matter for galaxy rotation curves and so forth. But now there is.
For a long time, there were some who dismissed the supernova evidence for the accelerating Universe on the basis of "grey dust." Those of us who did the supernova work looked for normal dust by looking at the colors of the supernovae. Just as the sunset is red, light that shines through interstellar dust is reddened. If the supernovae were too red compared to the standard colors of a Type Ia supernova, we knew we were seeing them through dust, and could either omit them from the data set, or correct for the dust. (Of course, generally we did both, to ensure consistency-- most of the papers include multiple different fits to the cosmologies using different subsets of the data and different correction procedures, all as part of controlling systematic errors.)
However, what if there was dust with grains big enough that they would dim without reddening? In that case, it would make the SNe at z=0.5-0.7 look too dim, but not too red. This was the main serious challenger (other than supernova evolution, which would have had to be fairly contrived) to the supernova evidnece for the accelerating Universe.
Many things have happened. First, Adam Riess and his co-workers have observed supernovae at z>1 where the Universe was decelerating, and have seen that deceleration; grey dust would have mimiced ever more acceleration. Second, the limits on grey dust got ever more severe that the grey dust models were becoming more contrived, to the point that some found them more aesthetically outlandish than the notion of an accelerating Universe. Third, corroborating evidence -- mass denstiy from clusters plus flatness from the CMB -- supported the notion of a positive cosmological constant.
Grey dust was worth thinking about. But it's not any more. We could probably keep tweaking with the models for the nature and cosmological evolution of grey dust to match the supernova Hubble Diagram, but at some point tuning your model like that makes less sense than just accepting the more direct straightforward view.
-Rob
These guys are good cosmologists, although the press release suffers from the usual lack of precision. They are explicating something that was actually discovered by Skordis et al. last year (astro-ph/0505519), namely that Bekenstein's relativistic version of MOND does a surprisingly good job at fitting the power spectrum of large-scale structure.
However, the caveats are significant. First, you absolutely need "dark matter" in this model (as well as dark energy), and almost as much of it as you need in ordinary CDM. The claim is that the dark matter can be "hot" (neutrinos will do the job), and hence relevant for clusters but not galaxies, which are then explained by the modification of gravity. As Rob points out, sure you can do this, but at some point people begin to lose interest.
Second, there is a hidden point that isn't emphasized enough -- the reason why Bekenstein's theory can fit the power spectrum is because it has new dynamical fields (a scalar and a vector) that act as sources for the gravitational potential. Now, there's a name we often attach to new dynamical degrees of freedom that don't couple directly to photons but act as sources for the gravitational potential -- "dark matter"! (See this talk.) You can call this "modified gravity" if you want, but really it's quite similar in spirit to (although much more complicated than) an ordinary model of new dark particles.
Sean Carroll wrote:
Hah!
Evidence is pretty good that "dark" mass makes up 23.87% of the total, as predicted. "Dark energy" is an entirely different animal.
Search beyond Dark Matter- see COSMOS PRIMER
http://vidyardhi.sulekha.com/blog/post/2007/02/cosmology-vedas-interlin…
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